Stents are endovascular prostheses which may be used for the treatment of stenoses (vascular constrictions). Stents have a base body in the form of a hollow cylindrical or tubular base lattice which is open at both longitudinal ends of the tube. The tubular base lattice of such an endoprosthesis is inserted into the vessel to be treated, and is used to support the vessel.
Such stents or other endoprostheses frequently contain metallic materials in their base body. The metallic materials may form a biodegradable substance which may also contain polymeric biodegradable materials.
The term “biodegradation” refers to hydrolytic, enzymatic, and other metabolic chemical degradation processes in the living organism which are primarily caused by the bodily fluids which come into contact with the endoprosthesis, resulting in gradual dissolution of at least large portions of the endoprosthesis. The term “biocorrosion” is often used synonymously for “biodegradation.” The term “bioabsorption” includes the subsequent absorption of the degradation products by the living organism.
Suitable substances (base substances) for the base body of biodegradable endoprostheses may be composed of a number of materials. Examples of suitable polymeric compounds include polymers from the group including cellulose, collagen, albumin, casein, polysaccharides (PSAC), polylactide (PLA), poly-L-lactide (PLLA), polyglycol (PGA), poly-D,L-lactide-co-glycolide (PDLLA-PGA), polyhydroxybutyric acid (PHB), polyhydroxyvaleric acid (PHV), polyalkyl carbonates, polyortho esters, polyethylene terephtalate (PET), polymalonic acid (PML), polyanhydrides, polyphosphazenes, polyamino acids, and the copolymers thereof, as well as hyaluronic acid. Depending on the desired properties, the polymers may be present in pure form, derivatized form, in the form of blends, or as copolymers. Metallic biodegradable materials are based, for example, on alloys of magnesium, iron, zinc, and/or tungsten.
The present invention relates in particular to stents or other endoprostheses having a base body whose material contains a metallic substance. In addition to the biodegradable materials named, noble metals such as platinum, iridium, gold, tantalum, yttrium, zirconium, ytterbium, or the alloys thereof represent suitable metallic substances.
It is known to provide stents with functional elements which have a different material composition, at least in a portion of their volume, compared to the material of the base body. These functional elements are used, for example, to determine the position of a stent in the body or for the release of medicaments.
The position of a stent is often determined by use of imaging methods, for example an X-ray device. Since the materials used for the base body of stents generally absorb only moderate amounts of X-ray radiation, i.e., they are X-ray permeable and/or radiolucent, stents are frequently provided with so-called markers as functional elements which contain a material that absorbs X-rays and/or other electromagnetic radiation (referred to below as radiopaque or X-ray opaque material) more strongly than the material of the base body.
U.S. Pat. No. 6,355,058 B1 describes a stent in which radiopaque markers in the form of particles are contained in a polymeric binder. The binder is distributed (dispersed) on the surface of the stent. Such a distribution of radiopaque particles generally does not provide for sufficient density of these materials, and therefore the radiopacity is too low for many applications.
U.S. Pat. No. 6,293,966 B1 discloses a stent having radiopaque marker elements which on their distal and proximal ends have a C-shaped element which in each case forms an essentially spherical receptacle. Marker elements having spherical end sections are inserted into these receptacles. The spherical end sections are affixed, by means of a positive fit and optionally by means of a weld connection, in the receptacles formed by the C-shaped elements.
DE 698 36 656 T2 illustrates and describes a bioabsorbable marker having radiopaque components for use on an implantable endoprosthesis such as a stent. The bioabsorbable radiopaque markers have, for example, porous sections filled with radiopaque material. A marker is also described which has hollow, cavity-like, porous sections filled with radiopaque material. The prior art also discloses a marker which is designed as an oblong element in the manner of a filament which is looped around sections of the implantable endoprosthesis.
For stents having a base body composed of a metallic material, when functional elements, for example radiopaque elements made of gold or silver, are provided on the base body of the stent the problem of contact corrosion occurs at the contact region between the material of the base body and the material of the functional element. This results in destruction of the stent, or separation of the functional element from the stent structure, so that the endoprosthesis can no longer perform its function, or may not be able to be found. The above-described endoprostheses from the prior art provide no solution to this problem.